Synergies of Cutting Air Pollutants and CO₂ Emissions by the End-of-Pipe Treatment Facilities in a Typical Chinese Integrated Steel Plant

Round 1

Reviewer 1 Report

Overall the manuscript is well written, and within the scope of the journal. I enjoy reading it. Below are my comments:

 

1. The authors present Figure 1 and Figure 2, with very little discussions of the figures. If these data are worth to be presented in figures, I expect they are important to the overall theme and deserved more discussions in the main text.

 

2. The authors mention in the Introduction that carbon oxidation rate is a major uncertainty in the EF method, while in the calculation, it appears as a parameter in the carbon balance method. Please explain.

 

3. Are there data and analysis available for nitrogen oxides (NOx) emission reduction from the plant and its synergy with CO2 emissions upon applying the end-of-pipe treatment?

 

4. Line 17: to account for

 

5. There are some formatting issues, such as the indentation is not consistent across the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review of manuscript "Synergies of cutting air pollution..." by Tang et al

Summary- This paper presents a "carbon balance" method of quantifying CO2 emissions from generic steel processing/production facilities in China, and applies this method to proposed "end of pipe" pollution control technologies at these industrial facilities. The authors claim the carbon balance method produces "greater accuracy" in estimating CO2 emissions. I am not a engineer familiar with detailed aspects of steel production, but an air pollution scientist, and found that this paper was really confusing. The title refers to "synergies" which in english implies a "win-win" type of relationship. Therefore I was expecting the authors to show that reducing local PM and sulfur pollution emissions can happen together with reductions of CO2 emissions. However, the opposite appears to be the conclusion of this paper: reducing SO2 and PM pollution INCREASES CO2 emissions. Is this correct? In which case the authors should not use the word synergy. As described below, it seems that the biggest "finding" of this paper is that estimating CO2 emissions from iron and steel production has a large uncertainty (factor of 4 in some specific process areas!), and the "carbon balance" method is somehow more accurate. This is never demonstrated or clearly explained. Why are the IPCC methods so grossly inaccurate? This paper needs significant revisions before publication. The third abstract claim about there being huge CO2 emissions that can be somehow reduced seems vague and unsupported by the content of the article.

Some detailed points that need to be addressed:

1) Fig 2 shows a vague measure of SO2 and particulate matter emissions as a percent ratio? A ratio of what? SO2 from steel divided by SO2 emissions from everything? Since this paper is discussing Iron and Steel Industry (ISI), please just show ISI emissions directly.

2) The authors describe three methods of quantifying CO2 emissions: 1) Emission Factors; 2) Carbon balance; 3) Life-cycle analysis. The authors claim that the use of EFs are inaccurate due to different "carbon oxidation" rates in different industrial or geographical areas. Unless substantiated, the CO2 emissions will be proportional to the carbon content of any fossil fuel used in the industrial process. If Chinese coal has a carbon content that is significantly different from coal in other places of the world, that can and should be documented here. As mentioned below, Since the authors are trying to defend a new CO2 emission accounting method as being more accurate, this needs to be clearly explained in words that even a non-engineer or expert would understand.

3) Fig 3 shows two "scopes", the only difference being the use of electric power plants outside scope 1. This seems unnecessary? What is the engineering difference between an onsite ISI electric power plant and "offsite" (Scope 2) "purchased electricity? both are produced in a machine powered by a fossil fuel. Is it just the mix of fossil fuels used in generating electricity? E. g., the purchased power could be hydroelectric-generated power?

4) Fig 3, scope 1 has Iron ore, limestone/dolomite, and fossil fuel included as "boxes" which are really externally imported into the ISI facility. Are the CO2 emissions associated with importing or transporting these raw materials to the ISI plants included in this analysis? Usually the geographical location of the ISI facility is chosen to optimize or minimize transportation costs associated with one or all of these raw materials. If the ISI facility is located far from iron ore, coal, or limestone resources, will CO2 emissions be higher according to this analysis?

5) The text near line 197 mentions ammonia as a desulfurization agent, but there is no NH3 input shown anywhere in Fig 3. Either include it as an input, or remove the discussion of its use here.

6) line 230 uses confusing variable names. "t" refers to tons of pollutant reduced from the exhaust stream, but in the next phrase, the same letter "t" refers to the "time" (hours) that the power plant operates. Please use "h" rather then "t" when referring to hours, just to avoid confusion.

7) in line 280, I'm confused about what appears to be excess electricity production/use of 848 mWh, (8% of total ISI electricity use)? So this ISI actually GENERATES electricity for the outside grid? If so, this seems to overly complicate the picture. So not only is this steel plant producing steel, it is also producing electricity for use outside the ISI? In which case the outside power plants can reduce their use of fossil fuel in their generation?

8) Table 3. The LARGE absolute and relative discrepancy in CO2 emissions involved in iron making (factor of 4!) between the EF and Carbon balance methods needs to be explained in a simple physics-engineering manner that readers like this reviewer can understand. The even larger relative difference in CO2 emissions for steel making (>factor of 30!, although small absolute numbers) also needs to be explained. The explanation given on line 330-331 that CO2 produced in the integrated steel enterprise is "often used comprehensively" is vague and meaningless from a technical standpoint. How does one make iron using 70% less carbon fuel? This is the major finding of this paper, and if the authors cannot clearly explain this large "improvement" in accuracy, the paper isn't publishable.

9) Fig. 4 shows HUGE sequestration of CO2 due to coking and iron making? Presumably this is due to the large sequestration of CO2 by something in flue desulfurization? If the authors of this paper are showing readers that Chinese ISI plants are sequestering large fractions of the CO2 emitted burning fossil fuels, that needs to be the headline of this paper: They've figured out how to sequester CO2!

10) After re-reading the text numerous times I have difficulty understanding what the three categories shown in Fig 4 represent: Product, Process, and Fuel. In the coking process, how does CO2 emissions result in a large negative emissions in the "product". I understand coking is a process where coal is heated/cooked without oxygen to produce a higher energy content coke product. How is CO2 sequestered in that process or product?

11) Line 498: In the paragraph above this, the authors note that CO2 emissions increase by adopting "end-of-pipe" removal of particulates and SO2 from pollution emission streams. However, the closing statement notes that there is big room for reducing CO2 emissions by adopting these pollution removal processes. This sounds inconsistent. How can adopting something that INCREASES CO2 emissions lead to lower CO2 emissions? Maybe this is just a english wording problem. Maybe what the authors are trying to say is that they would hope that someday CO2 emissions could be reduced by adopting more CO2 efficient methods of removing PM and SO2 pollution

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The paper develops an analysis of analyses of reduction of local air pollutants and impacts on GHG emissions of end-of-pipe treatment facilities in Chinese integrated steelmaking plants. The carbon balance method is exploited in order to calculate the production and emissions of local air pollutants and CO2 in each process.

The topic of the paper is interesting and fits well the aims and scope of the Journal. The adopted methodologies are well consolidated and not new, but they are correctly applied and the issue faced by the paper is worth of investigation. Although the elements of novelty of the proposed study are limited, the proposed analysis is consistent and technically sound.

However, the paper presents some weaknesses, which prevent its publication in the present form.

The state of the art analysis related to air pollution and CO₂ emissions abatement in the steel sector is quite poor and mostly limited to papers of Chines authors. Even if one wants to focus on the Chinese scenario, the analysis of literature need to be improved by including at least the following recent papers:

• Zhang, F. et al.: Estimation of abatement potentials and costs of air pollution emissions in China, (2020) Journal of Environmental Management, 260
• Li, H. et al: Study on an implementation scheme of synergistic emission reduction of CO 2 and air pollutants in China's steel industry, (2019) Sustainability, 11 (2).
• Zhang, Q., Energy and resource conservation and air pollution abatement in China's iron and steel industry, (2019) Resources, Conservation and Recycling, 147, pp. 67-84

The results of the proposed analysis are overall well presented and extensively discussed. However, when evaluating the CO₂ emissions, it would be interesting to consider also the possible effect of improving the management of the off-gas network as well as its retrofit through advanced optimization systems. For relevant studies one can refer, for instance, to:

• Maddaloni A. et al.:"Multi-objective optimization applied to retrofit analysis: a case study for the iron and steel industry," Applied Thermal Engineering, Vol. 91, 2015, pp. 638-646.
• Zhao X. et al. A review on the optimal scheduling of byproduct gases in steel making industry, (2017) Energy Procedia, 142, pp. 2852-2857.

One weak aspect of the proposed analysis lies in the fact that only local air pollutants are considered, while neglecting other kind of pollutants (e.g. emissions in local water bodies and soil contamination), which might be generated by related to end-of-pipe facilities. However, these kind of pollutants are still very relevant for the health and well-being of the local community living in the surrounding of the steelmaking sites. This concept should be more deeply elaborated in the paper.

The English language absolutely needs to be revised and improved, by eventually exploiting the support of a professional proofreader, as there are a relevant number of incorrect expressions that need to be eliminated. See the list below for some examples extracted from the first pages (but there are also many other inconsistencies):

• Page 1, row 19 (abstract): please replace “…compared with…” with “…compared to…”.
• Page 1, rows 21-22 (abstract): the following sentence is unclear “Since huge CO₂ emissions produced by adopting end-of-pipe pollution treatment facilities, there is still a room for….”. I guess it might be as follows: “Since huge CO₂ emissions ARE produced by adopting end-of-pipe pollution treatment facilities, there is still ROOM for….”.
• Page 1, rows 31-32: the following sentence is unclear and not correct: “…and it is also the focus of research for controlling GHGs and local air pollutant emissions.” I guess it might be as follows: “…and research is focused on control of GHGs and local air pollutant emissions.”
• Page 2, row 50: please replace “near completion” with “close to completion.”
• Page 2, row 55: please replace “is generally applied” with “are generally applied.”
• Page 2, row 56: please replace “which are the commonly used as industrial dedusting facilities” with “which are the most commonly used industrial dedusting facilities”.

Finally, a list of the used acronyms and symbols should be added in order to improve the readability of the paper.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript reports on the emissions reductions associated with end-of-pipe abatement facilities for steel production in China. 

In the main the manuscript is satisfactorily written and presented well.  The comments below aim to be constructive.

page 21 line 28 ‘Since local air pollutants have a homology with greenhouse gas (GHG) production’ – can the authors explain briefly what ‘local air pollutants’ consist of, why they are a problem and why they are homologous with greenhouse gas emissions.  

line 36 and Figure 2 – what particulate matter (PM) is being referred to?  PM10, PM2.5? needs clarification. 

The study considers PM (unknown), SO2 and CO2.  Suggest the authors clarify why it was only limited to these pollutants and other emissions earlier in the manuscript e.g. nitrogen oxides were not assessed but reference is made on line 597 ' the target plant in this study was not equipped with any end-of-pipe facilities for denitration'. 

CO2 emissions provide data for the LCA impact category greenhouse gas emissions, SO2 should be discussed in terms of other LCA inventory impacts  e.g. acidification - any reason why was this not included?  Reference is made to power consumption, any reason why energy was not included?

line 90 ‘This study used the emission factor method, carbon balance method to calculate the production’ – should reference to LCA be included here also given that is referred to in the same sentence as these two methods on line 81?  Does the study refer to CO2 emissions or CO2 equivalent emissions?

line 102 ‘This research selected 2 scopes for consideration. In scope 1, the production, reduction and emission of local air pollutants and CO2 were studied’ suggest local air pollutants is stated as PM and SO2 throughout

line 419 ‘Figure 5. Distribution of the emission sources under two carbon-related parameters.’ clarify which emission is being referred to

line 469 ‘we could hardly determine if the source of power consumed’ replace with ‘it was not possible to determine if the source…’

would be useful to show the emission sources at various phases in the process displayed as per tonne of steel with and without abatement facilities to allow a standardised comparison with other LCA studies of this nature – can this be included with the total emissions reductions data in Table 5 or used to replace the data in Figure 6 which displays proportion of CO2 emissions and seems to repeat the data in Table 5.  Same comment for the data in Table 6.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The Authors improved the paper according to the suggestions of the reviewers. I have no further observations on the paper, which is now suitable to publication.